frequency of a true count 8 decks blackjack

I read online that the frequency of a +5 true count with 8 decks good penetration is 3.45%
Does anyone know the formula, I'd like to plug in other numbers

Is a true count where 10-K gives a value of -1 and 2-9 is +1? I don't remember how aces factor in. What goes good penetration mean? I think without knowing the exact number of cards cut from the 8 decks makes this problem very difficult.